RFR: 8230092: ZGC: Consolidate ZBackingFile, ZBackingPath and ZPhysicalMemoryBacking on Linux

[Per Liden]

After JDK-8230090, we can move ZBackingFile, ZBackingPath and 
ZPhysicalMemoryBacking to os/linux/gc/z rather than having them 
duplicated in os_cpu/linux_{x86,aarch64}/gc/z.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230092
Webrev: http://cr.openjdk.java.net/~pliden/8230092/webrev.0

Testing: Builds on linux_x86 and linux_aarch64, passes dacapo

/Per